In wavelength division multiplexed (WDM) optical communication systems several optical channels may be transmitted on a single optical fiber with narrow channel spacing, for example, less than 25-35 GHz. To mitigate crosstalk between channels, it may be important to accurately and reliably position the frequency for each channel within a small range around a fixed frequency grid. The frequency grid might be an absolute grid, e.g., based on International Telecommunication Union (ITU) standards, or a relative grid determined, for example, by one or more filters.
To position channel frequencies, a wavelength monitor may be used. In general, it may be desirable that a wavelength monitor position channel frequencies without creating significant system transmission penalties. It may also be desirable for a wavelength monitor to co-operate with combinations of various equipment generations and configurations. For example, it may be useful for a wavelength monitor to establish channel frequencies in mixed systems with signals of different modulation formats, such as On-Off-Keying (OOK) and differential phase-shift-keying (DPSK).